The video preview on Blender is suddenly just a still image. How do I make it actually show a preview of the video?
If the video has not yet been rendered, and you only want to play the preview then you will press  Alt + A . If it has been rendered, then you will use  Ctrl  + F11. I see that you're in the uv/image editor also, switch to 3D view to see your preview. You can do this quickly by changing windows with the menu on the far left of the header on the bottom of the window.